Showing papers by "Thomas Fromherz published in 2003"
TL;DR: In this article, the 2D periodic arrays of Ge islands were realized on prepatterned Si (001) substrates by solid-source molecular-beam epitaxy, and atomic-force microscopy images demonstrate that the Ge islands are formed in 2D laterally ordered pits of patterned substrates.
Abstract: Two-dimensional (2D) periodic arrays of Ge islands were realized on prepatterned Si (001) substrates by solid-source molecular-beam epitaxy. Atomic-force microscopy images demonstrate that the Ge islands are formed in the 2D laterally ordered pits of patterned substrates. The 2D periodicity of the substrate pattern is replicated throughout a stack of Ge island layers by strain-driven vertical ordering. Photoluminescence spectra of the ordered Ge islands show well-resolved peaks of the no-phonon signal and the transverse-optical phonon replica. These peaks are observed at nearly the same energy as those of random Ge islands deposited under the same conditions on unpatterned Si substrates.
115 citations
TL;DR: In this paper, a large tunability of the photoresponse peak wavelength (from 5.2 to 3.2 μm) by an externally applied electric field is observed.
Abstract: Photocurrent spectroscopy has been performed on doped Si/SiGe valence band cascade injector structures in the mid-infrared spectral region. A large tunability of the photoresponse peak wavelength (from 5.2 to 3.2 μm) by an externally applied electric field is observed. The tunability of the photoresponse is a consequence of an electric-field-induced transfer of holes from the deepest to the shallowest quantum well of the injector sequence. Depending on the bias voltage, dark-current-limited peak detectivities of D*=1×109 cmHz/W (peak wavelength 5 μm at −4 V bias) and of D*=1.3×109 cmHz/W (peak wavelength 3.2 μm at 5 V bias) are obtained at a temperature of 77 K.
7 citations
TL;DR: In this article, the authors investigated the intersubband line positions in Si/Si1−xGex(x=80%) heterostructures which are deposited by MBE on a virtual substrate of relaxed SiGe containing 50% of Ge.
Abstract: This study follows up our previous investigation of the valence band (VB) intersubband emission from quantum cascade structures grown lattice matched on Si substrates. Here, Si/Si1−xGex(x=80%) heterostructures are investigated which are deposited by MBE on a virtual substrate of relaxed SiGe containing 50% of Ge. TEM analysis reveal flat and abrupt interfaces for structures grown at temperatures Tgrowth≈300°C. Intersubband absorption and photoluminescence emission manifest well-defined interfaces and good material quality. The observed intersubband line positions are found to be in good agreement with k·p model calculations for the VB. This is in contrast to the observed type II no phonons recombination which is found at consistently lower energy than expected. Finally, electrically excited intersubband emission from a strain compensated cascade structure containing three periods is presented.
4 citations
01 Jan 2003
TL;DR: In this article, the concept of QCLs can also be applied to indirect bandgap materials since it relies on intersubband optical transitions for which the nature of the bandgap does not play any role.
Abstract: Quantum cascade lasers (QCLs) based on III-V materials such as InGaAs/InAlAs [1] or GaAs/AlGaAs [2] have attracted an increasing attention since the first experimental demonstration in 1994. These convenient emitters have been successfully operated in the 3.4-24 μm range, and also recently in the terahertz range [3]. The concept of QCLs can also be applied to indirect bandgap materials since it relies on intersubband optical transitions for which the nature of the bandgap does not play any role. The Si/SiGe material system is particularly a candidate of interest, because of the possible compatibility with the well established Si integrated circuit processing.
1 citations